Heat exchanger for high flow rate infusion

ABSTRACT

A heat exchanger has a laminar fluid flow path receivable between the heating plates of a high flow rate infusion unit to which heat is conducted by contact with the heating plates. A bubble trap and a valve are integrated with the heat exchanger. The bubble trap collects air from the infusate exiting the laminar flow path, and includes an air vent in contact with the infusate that vents the air from the bubble trap. The valve shuts off the flow of infusate if air is detected in the bubble trap.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application contains subject matter related to the followingpatent applications, all assigned to the assignee of this application:

U.S. patent application Ser. No. 10/214,966, filed Aug. 8, 2002, for“Fluid Warming Cassette with a Tensioning Rod”, published as US2004/0026068 A1 on Feb. 12, 2004;

U.S. patent application Ser. No. 10/397,942, filed Mar. 25, 2003, for“Fluid Warming Cassette and System Capable of Operation under NegativePressure”, published as US 2004/0190885 A1 on Sep. 30, 2004;

U.S. patent application Ser. No. 10/822,580, filed Apr. 12, 2004, for“Intravenous Fluid Warming Cassette with Rails and a Stiffening Member”;

U.S. patent application Ser. No. ______, filed Apr. 24, 2007, for “HighFlow Rate Infusion Unit and Heat Exchanger”; and,

U.S. patent application Ser. No. ______, filed Apr. 24, 2007, for“Bubble Trap for High Flow Rate Infusion”.

The assignee of this application now owns the following issued U.S.patents containing subject matter related to the subject matter of thisapplication: U.S. Pat. Nos. 5,807,332; 6,464,666; 6,535,689; 6,775,473;and 7,010,221.

The assignee of this application now owns European Patent 1 159 019,granted Nov. 6, 2002 for “IV Fluid Warming System with Detection ofPresence and Alignment of Cassette”, which has been validated inGermany, France, Great Britain, Ireland, and Monaco.

See PCT application PCT/US20000/02630, filed Feb. 2, 2000 for “PressureTolerant Parenteral Fluid and Blood Container for a Warming Cassette”,publication WO 01/26719, Apr. 19, 2001, filed by the assignee of thisapplication.

BACKGROUND

The subject matter relates to heat exchanger for a high flow rateinfusion unit that pressurizes and warm fluids for infusion into a bodyat pressures equal to or exceeding gravity.

Infusion relates to the introduction of a fluid into a body, usually,although not necessarily, into vasculature. A fluid that is infused intoa body may be termed an “infusate”. Such fluids may include, forexample, blood, blood products, and solutions such as saline,antibiotics, and medications.

The combination of low operating room temperatures and theadministration of anesthetics which inhibit a patient's thermoregulatoryfunction leads to hypothermia during surgery. As is known, perioperativehypothermia can produce adverse outcomes such as surgical woundinfection, extended hospitalization, and blood loss. See Sessler D I:Complications and Treatment of Mild Hypothermia. ANESTHESEOLOGY 2001;95:531-543. Prevention or mitigation of hypothermia, particularlyperioperative hypothermia, is thus a key clinical factor for successfultreatment outcomes.

Hypothermia may be accelerated by infusion of fluid, especially if thefluid is refrigerated. For example, Sessler indicates that a unit ofrefrigerated blood or a liter of crystalloid solution at roomtemperature decreases the mean body temperature of adults byapproximately 0.25° C. But patients suffering from serious trauma mayrequire rapid infusion of large amounts of fluid, which can cause asharp and sudden loss of heat in the body core, leading to a drop inmean core body temperature. In order to prevent or mitigateinfusion-caused heat loss in a trauma patient, the infusate is oftenheated as it is administered.

Warming fluid prior to infusion into a human or animal body is known.See, for example the intravenous fluid warming systems and appliancesdescribed in the cross-referenced patent documents. See also the Ranger®blood/fluid warming system and products described at www.arizant.com,the web site of Arizant Healthcare Inc. The Ranger® blood/fluid warmingsystem includes a heating appliance and a heat exchanger capable ofbeing inserted into the heating appliance. Fluid flowing though the heatexchanger is warmed by contact between the heating appliance and heatexchanger, and then delivered intravenously to a patient. However, thedisclosed systems cannot meet all rates of infusate delivery needed fortreatment of trauma patients.

The technical challenges in heating a high volume of infusate deliveredat a relatively high rate, for example, at 30 liters per hour (30 L/hr),or higher, include uniform transfer of heat to the fast-flowing fluid,elimination of air from the fluid, and an infusion system constructionthat supports convenience and speed of operation.

Solutions to these challenges in the prior state of the art include aknown high speed infusion system that warms infusate by immersion of aheat exchanger in a warm water bath. A column of infusate flows throughthe heat exchanger, and the warm water bath heats the infusate as itpasses through the heat exchanger. The heat, the flow pattern and highflow rate of the infusate create bubbles in the infusate, which must beremoved before intravenous delivery in order to avoid formation of anair embolism in the patient being infused. This high speed infusionsystem includes a gas elimination device to collect bubbles from theinfusate, and a clamp to halt the flow of infusate if air is detected inthe infusate.

The known high speed infusion system is constituted of an appliance witha water heating and circulation system. The heat exchanger consists of apair of coaxial tubes, a smaller one disposed inside a larger one. Theinfusate flows through the annulus between the larger and smaller tube,and the heated water is circulated from the heater, through the innertube, and back to the heater. The heat exchanger is installed in theappliance where it must be reliably coupled to an infusate flow path andto a separate hot water flow path. The gas elimination device isseparate from the heat exchanger; and it is installed separately anddownstream from the heat exchanger. Infusate passes through the gaselimination device into a patient line for intravenous delivery to apatient. When a predetermined amount of air is detected in the gaselimination device, a downstream clamp is activated to pinch off thepatient line, thereby stopping the flow of infusate to the patient. Theheat exchanger and gas elimination device are discarded after each use,and new ones must be installed each time a patient is treated.

In this known high speed infusion system, the heat transfer mechanismposes a risk of an exchange of contaminants between the infusate and thewater used to deliver heat. This may occur when the barrier between thewater and the infusate is breached for some reason. The use of a warmwater bath as the heat transfer mechanism requires continuousmaintenance to keep the water clean and the pumping system operatingwith sufficient capacity. Air transported from infusate bags and bubblesgenerated from the infusate are collected and separated by the gaselimination device, and air is eliminated through a port in the device.At times, a large mass of collected bubbles can block the port, therebypreventing air from being vented. Then, the collected bubbles will causedetection of air that is not quickly vented, and the clamp will beactivated. In such a case, the system can be restarted only afterclearing or replacing the gas elimination device. In this known system,set up preceding each use requires separate handling and installation ofthe heat exchanger, the gas elimination device, and the length ofpatient line that is led through the clamp.

There is a need for a heat exchanger for high flow rate infusion thateffectively transfers heat to a rapidly-flowing infusate without risk ofcontaminant exchange between the infusate and a heat transfer fluid.Another desirable advance would reliably eliminate air from the infusatewithout blocking an air vent. System set up would be improved byreduction of the number of devices required to be installed each timethe system is used.

SUMMARY

A heat exchanger to conductively heat infusate flowing therethrough at ahigh flow rate includes a laminar flow path and a bubble trap in fluidcommunication with the laminar flow path.

A heat exchanger has a laminar fluid flow path receivable between theheating plates of a high flow rate infusion unit to which heat isconducted by contact with the heating plates. A bubble trap and a valveare integrated with the heat exchanger. The bubble trap collects airfrom the infusate exiting the laminar flow path, and includes an airvent in contact with the infusate that vents the air from the bubbletrap. The valve shuts off the flow of infusate if air is detected in thebubble trap.

A heat exchanger embodiment constituted of a flat, elongate warmingcassette with a fluid container defining a laminar fluid flow path isslidable in a heating unit between a seated position where the fluidcontainer is in heat-transferring contact with heating plates and anextracted position outside of the electrical heating unit. The warmingcassette includes a housing attached to the fluid container. The housingcontains a bubble trap and a valve. The bubble trap is disposed in fluidcommunication with the laminar flow path to collect air from infusateflowing out of the fluid path. The bubble trap includes an air vent incontact with the infusate that vents air from the bubble trap. Thevalve, disposed in fluid communication with the bubble trap, has an openstate permitting infusate to flow out of the warming cassette and aclosed state blocking infusate from flowing out of the warming cassette.

The bubble trap includes a flow expansion chamber to collect largebubbles, a recirculation chamber to collect large to medium bubbles, alaminar flow chamber where air is released and detected, and an outletchamber where the infusate exits the heat exchanger.

The heat exchanger is constructed for sensing the presence and level ofair in infusate flowing through the bubble trap in order to control theflow of infusate out of the bubble trap in response to detected levelsof air. Preferably, the infusion unit controls the state of the valve inresponse to detected levels of air in order to permit or preventinfusate to flow.

The unification of a laminar flow path, bubble trap, and shut off valvein an integrated heat exchanger construction yields a single, easilyhandled appliance that simplifies setup and operation of infusatewarming, bubble management, air elimination, and safety shut off forhigh flow rate infusion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a high flow rate infusion unit and aheat exchanger in an extracted position with respect to the infusionunit.

FIG. 2 is a perspective view of the high flow rate infusion unit withthe heat exchanger in a seated position with respect to the infusionunit.

FIG. 3 is an enlarged perspective view of the top of the high flow rateinfusion unit with two pressure-actuated infusers, in which one pressureinfuser is opened to receive an intravenous (IV) bag.

FIG. 4A is a side sectional view of a pressure infuser with a full IVbag mounted therein against a deflated bladder. FIG. 4B is a sidesectional view of the pressure infuser of FIG. 4A with the IV bag emptyand the bladder inflated.

FIG. 5 is an exploded view of a warming cassette, in perspective.

FIG. 6 is a plan view of the assembled warming cassette.

FIG. 7 is a side elevation view of an electrical heating unit of thehigh flow rate infusion unit with the warming cassette in the seatedposition with respect to heating plates of the electrical heating unit.

FIG. 8 is an exploded top view of the electrical heating unit with thewarming cassette in the seated position between the heating plates.

FIG. 9 is a sectional view of the electrical heating unit taken alongline 9-9 of FIG. 7 with the warming cassette in the seated positionbetween the heating plates.

FIG. 10 is an enlarged view of the front face of a housing of thewarming cassette of FIG. 6 showing a bubble trap, sensor couplers, and avalve.

FIG. 11 is sectional view of the housing of FIG. 10 taken along line11-11 of FIG. 10.

FIG. 12A is a perspective view of the front face of the housing of FIG.10, partially disassembled. FIG. 12B is a perspective view of the backface of the housing of FIG. 10, partially disassembled and shown withrespect to a mounting flange in the high flow rate infusion unit withsensors and an actuator partially disassembled therefrom.

FIG. 13 is an enlarged sectional view showing details of an air vent.

FIG. 14A is an enlarged plan view of a sensor coupler piece. FIG. 14 Bis a longitudinal cross section of the sensor coupler piece.

FIG. 15 is a partial side cross sectional view of an upper portion ofthe housing of FIG. 10 showing engagement between a sensor and a sensorcoupler.

FIG. 16 is an enlarged perspective view of a bubble trap hydrophilicscreen.

FIG. 17 is an enlarged side sectional view of a valve in fluidengagement with the bubble trap in the housing of FIG. 10.

FIG. 18 is an enlarged perspective view of the side of the high flowrate infusion unit partially cut away to illustrate constructiondetails.

FIG. 19 is an enlarged perspective view of the front of the high flowrate infusion unit with the warming cassette partially inserted into thehigh flow rate infusion unit.

FIG. 20 is an enlarged perspective view, partially cut away, of thefront of the high flow rate infusion unit with the warming cassetteseated in a heating unit thereof.

FIG. 21 is an enlarged perspective view of the side of the high flowrate infusion unit with the warming cassette seated therein and with thewarming cassette and infusion unit partially cut away to illustrateconstruction details.

FIG. 22 is a block diagram representing an electronic control subsystemfor the infusion unit of FIGS. 1 and 2.

FIG. 23 is a schematic diagram representing a pneumatic subsystem forthe infusion unit of FIGS. 1 and 2.

FIG. 24 is a flow diagram illustrating operation of the high flow rateinfusion unit in conjunction with the heat exchanger.

FIG. 25 is a schematic diagram of a sensor in operational engagementwith the bubble seen in FIG. 10.

FIG. 26 is a schematic diagram of the bubble trap seen in FIG. 10 with ahydrophobic membrane for venting air.

FIG. 27 is a flow diagram illustrating a logic control mechanization tocontrol flow through the bubble trap illustrated in FIG. 26.

FIG. 28 is a schematic diagram of a second bubble trap embodiment.

DETAILED DESCRIPTION

In this detailed description, a heat exchanger including a laminar flowpath for a high flow rate infusion unit is described. A high flow rateis a flow of infusate through a patient line at a rate that issufficient to administer a large amount of infusate quickly to a person.For example, a high flow rate infusion system may administer blood to atrauma patient at a rate of 30 liters per hour (30 L/hr), or higher,measured through a line connected intravenously to the patient (a“patient line”). A “laminar flow path” is a thin, relatively flat,non-sinuous space through which a sheet of infusate can flow from aninlet port to an outlet port.

The novel designs and embodiments to be described provide a number ofbenefits with respect to previously known high flow rate infusionsystems. Infusate is heated by direct contact between the heatgenerating mechanism and a heat exchanger, thereby eliminating anintermediary medium (such as water) to transport heat from a heater tothe heat exchanger. This mode of heat transfer may be referred to as“dry heat” because it does not use water, or another fluid. An exemplaryheat exchanger construction unifies a unidirectional, laminar flow pathwhere infusate is heated, a bubble trap that continually vents air fromthe infusate, and a valve to regulate infusate flow. Bubbles areseparated and collected from infusate and air is eliminated through avent in contact with the infusate, which reduces shut downs caused bybuild up of bubbles, thereby ensuring uninterrupted infusate flow forlonger periods of time than the previously known high flow rate infusionsystems. The unified construction of the heat exchanger yields a single,easily handled appliance that simplifies setup and operation of heatexchange, air elimination, bubble entrapment, and safety shut off for ahigh flow rate infusion unit.

High Flow Rate Infusion System

Refer now to FIGS. 1 and 2 which illustrate a high flow rate infusionsystem including a high flow rate infusion unit 10 and a heat exchanger12. The infusion unit 10 has a kiosk or tower construction with a casingincluding an upper section 16 with dual, pressure-actuated infusers 18,a neck 20 extending from the upper section 16, and a pedestal 22supporting the neck 20. Preferably, a wheeled support base 24 allows theinfusion unit 10 to be easily moved or repositioned on a floor or othersurface. A rack 26 is supported above the upper section 16 by a shaft 28slidably retained in the upper section 16. Bags of infusate may be hungon the rack 26 as shown. The longitudinal axis of the infusion unit 10is generally perpendicular to the surface on which it is supported.Electronics for operating the infusion unit 10 are contained in the neck20. A heating unit 27 constituted of resistively-heated plates iscontained within the pedestal 22. Sensors, actuators, and a pneumaticsystem for delivering pressurized air are distributed as needed betweenthe neck 20 and the pedestal 22. The pedestal 22 has a recessed surfaceportion 30 where a bezel 32 is mounted. The bezel 32 has an elongateopening or slot 34. As seen in FIG. 1, a mounting block 36 in therecessed surface portion 30 is disposed along one side of, andperpendicularly to, the bezel 32. Sensors 37 and 38 are mounted to andextend through the body of the mounting block 36 to a major surface 42thereof. A valve actuator 40 (best seen in FIG. 12B) mounted to a rearsurface of the mounting block 36 includes a piston 41 that operatesthrough the major surface 42.

The construction of the heat exchanger 12 includes a laminar flow paththrough which a broad sheet of infusate flows. In use, when the heatexchanger 12 is installed in the infusion unit 10, the laminar flow pathof the heat exchanger is sandwiched between a pair ofelectrically-operated heating plates, such that each side of the laminarflow path is in close heat-conducting contact with a respective one ofthe pair of heating plates. When the heating plates are operated, heatexchanged between the plates and the laminar flow path warms theinfusate as it moves through the laminar flow path. With reference toFIGS. 1 and 2, an exemplary construction of the heat exchanger 12 isillustrated. Preferably, the heat exchanger 12 may be constructed as anelongate, quadrilateral, generally flat or laminar warming cassette 60.The cassette 60 includes a distal end 61, a fluid container 62, and ahousing 64 with a hand grip 65. The fluid container 62 defines a laminarflow path 67 of the warming cassette 60. The cassette 60 includes aninput port 69 and an output port 71, each in fluid communication withthe laminar flow path 67. When viewed end on looking toward the distalend 61, the cassette has a thin, but relatively elongate aspect so as tobe slidably inserted into the slot 34 in the bezel 32 with the fluidcontainer sandwiched between and in heat-conducting contact with theheating plates, and slidably extracted therefrom.

In FIG. 1, the cassette 60 is shown extracted from the infusion unit 10;in FIG. 2, the cassette 60 has been inserted in the infusion unit 10,distal end 61 first, through the slot 34 into the electrical heatingunit 27, where the fluid container is disposed between and in contactwith the heating plates. Preferably, when the cassette 60 is insertedinto the slot 34, the longitudinal axis of the infusion unit 10 and amajor axis of the cassette 60 are generally aligned and parallel. Thus,when the cassette is received in the slot 34, it is oriented to bedisposed substantially vertically with respect to a surface supportingthe infusion unit 10. The cassette 60 is removed from the infusion unit10 by grasping the hand grip 65 and pulling the cassette upwardly, outof the slot 34. In most aspects, after infusion of a patient, a usedcassette 60 is extracted from the infusion unit 10 and processed formedically safe disposal. A new, unused cassette 60 is inserted into theinfusion unit 10 prior to commencing infusion of another patient.

With further reference to FIGS. 1 and 2, the infusion unit 10 isprepared for operation by placing a bag containing infusate into eitheror both pressure infusers 18, inserting the cassette 60 into the slot34, and connecting the bag or bags to the cassette 60 by IV tubing. AnIV tube set such as the Y tube set 73 is connected to each bag and tothe input port 69 of the cassette 60. The Y tube set 73 is conventionaland includes manually-operated means 74 in each branch of the Yconnected to a bag to pinch off the branch when the bag connected to itis not used. An IV tube 75 is connected to the output port 71 of thecassette 60 and is connected by known intravenous means to a patient.The IV tube 75 constitutes the “patient line” through which a flow ofwarmed infusate is delivered intravenously to a patient at a rate thatis sufficient to administer a large amount of infusate quickly to thepatient. For example, the rate may be 30 L/hr, or higher. The infusionunit 10 is then activated by means of controls operated by a user usingcontrol panel 77. ON/OFF control is afforded by way of control panel 79.

With further reference to FIGS. 1 and 2, when activation of the highflow rate infusion unit 10 occurs, electrical power is applied toresistively heat the heating plates and pressurized air is introducedinto an inflatable bladder in a pressure infuser 18. As the bladder 103inflates, it presses against the fluid-filled bag in the pressureinfuser 18, which forces the fluid into the IV tubing set 73. Thepressure against the bag is transferred to the fluid, forcing it to flowto and through the cassette 60 at a rate higher than that which wouldresult if it were flowing in response to gravity only. The infusateflows into the cassette 60 through the input port 69 and therethroughinto the laminar flow path 67 near the distal end 61. The infusate fansout into a thin laminar sheet and flows through the laminar flow path67, expanding the fluid container 62 so that it contacts and pressesagainst the heating plates. The infusate continuously absorbs heat fromthe heating plates as it flows. As the infusate approaches the housing64, the shape of the laminar fluid flow path 67 concentrates the warmedinfusate into a narrow, high speed stream that flows into the housing64, through a bubble trap 80 where bubbles are separated and collectedfrom the stream of infusate, and where air is vented through an air vent81. Passing through the bubble trap 80, the narrow, high speed stream ofwarmed infusate flows through a valve 82, out the output port 71, intothe patient line 75, through which it is administered intravenously to apatient.

With reference to FIGS. 3, 4A, and 4B, each pressure infuser 18 isconstructed to receive a full bag of infusate and to expel the infusatefrom the bag at a high rate of flow. Each pressure infuser 18 has a bodyconstituted of a rear shell 93 and an inner shell 95 fixed to the rearshell 93 and supported thereagainst by spacers 97. In the space betweenthe shells 93 and 95, a pneumatically controlled valve 99 and anelectronically controlled, three way pneumatic valve 100 are mounted tothe rear surface of the inner shell 95. A port 101 extending through theinner shell 95 connects the valve 99 to an inflatable bladder 103supported on the front surface of the inner shell 95. Each pressureinfuser 18 has a door 105 that swings on a hinge 107 mounted to the bodyof the pressure infuser 18. Each door 105 is held shut by an elongatereleasable latch 109 mounted to the body of a pressure infuser 18. Apair of spring retainers 111 is mounted to the body of each pressureinfuser 18 so as to extend into the space between a door and an innershell. The springs support bags when the doors open and aid dooropening.

With further reference to FIGS. 3 and 4A, the door 105 of a pressureinfuser 18 is opened, and a full bag B of infusate with a lower port Pis placed in the pressure infuser 18, such that the port P extendsdownwardly through a gap between the door 105 and the body of thepressure infuser 18. The bag B is retained against the front surface ofthe inner shell 95 by the pair of spring retainers 111 and by closingand latching the door 105. Preferably, the bag B has a construction thatis conventional for IV bags, although the design may be customized toaccommodate other design requirements. For a conventional construction,the bag B is connected to one line of the Y tube set 73 by a spike onthe end of the line that penetrates the bottom of the bag B through theport P. With reference to FIGS. 4A and 4B, infusate is pressurized andforced from the bag B, through the port P, when the bladder 103 isinflated by pressurized air provided by the two-way valve 99 through theport 101. Pressurized infusate flows out of the bag B through the port Pinto the line 73a, and therethrough to the heat exchanger 12. When thebag B is emptied, the setting of the valve 99 is reversed, and thebladder 103 is deflated by venting air from the bladder through the port101. The empty bag B may then be removed from the pressure infuser 18and replaced by another full bag.

The flow rate of the infusion system just described is established by,among other parameters, the viscosity of the infusate, the pressurecapacity of the pressure infusers 18, and the resistance to fluid flow.Infusate viscosity varies according to the nature of the fluid beinginfused. The rate of inflation of the bladders 103 and the relativesizes of the bladders 103 and infusate bags are the principaldeterminants of pressure capacity. The broad laminar flow path in thefluid container 62 reduces flow resistance, compared to previous heatexchanger designs based on a flat cassette, by elimination of curves,bends, and abrupt changes in flow direction. Tubing can be selected toprovide a range of flow resistance appropriate to the other factors andthe desired flow rate. Preferably, the high flow rate infusion system ofFIGS. 1 and 2 administers blood to a trauma patient at a rate of 30liters per hour (30 L/hr), or higher, when the pressure infusers 18 areoperated to pump infusate by inflation of the bladders 103. Of course,the infusate bags may be connected to a heat exchanger 12 installed inthe infusion unit 10 for flow of infusate through the infusion system ata lower pressure. In fact, infusate will flow without activating thepumping operation of the pressure infusers 18 at all, in which case,infusate will flow through the system by gravity. Thus, the infusionsystem of FIGS. 1 and 2 can provide warmed infusate at flow rates in therange of from 0 to at least 30 L/hr; in some instances, the infusionsystem can provide warmed infusate at a maximum flow rate exceeding 70L/hr.

Heat Exchanger

Infusate expelled from a pressure-activated infusate bag travels at ahigh flow rate through tubing connecting the bag to the heat exchangerin which it is warmed for administration to a patient. The heatexchanger is exemplified by a warming cassette construction adapted foruse in the infusion unit 10 of FIG. 1. In this description of thewarming cassette, the term “heat exchanger” is used to denote thewarming cassette, even though heat exchange occurs through the fluidcontainer and is only one function of the warming cassette. The warmingcassette 60 has an integrated construction that unites a heat exchangerin the form of the fluid container 62, with a bubble trap and shut-offvalve disposed in the housing 64. This construction enables the heatexchanger, bubble trap, and shut-off valve to be installed in andremoved from the infusion unit 10 in a single step. With reference toFIGS. 5 and 6, the fluid container 62 is a thin, quadrilaterally-shaped,fluid-tight pouch 150 formed by joining coextensive sheets of flexibleplastic material together by a pattern of fluid-resistant seals 151around the periphery of the pouch 150. Two semi-rigid plastic rails 153and 155 are positioned between the coextensive sheets and betweenelements of the seals 151 just inside of and parallel to the elongateedges 157 of the pouch 150. The rails 153 and 155 are sealed to thesheets of flexible plastic material by fluid-resistant seals 152, nearthe ends of the rails. The laminar flow path 67 is positioned betweenthe rails 153 and 155 and has an inlet 160 and an outlet 161. The rail153 has a straw like construction with a central passageway 162 thatopens through one end 163 of the rail 153 and extends to a groove 164terminated in a short longitudinal slot 165 near the opposing end. Theslot 165 opens through the side surface of the rail into the inlet 160to laminar fluid flow path 67. The rail 155 has a short centralpassageway 162 that opens into the outlet 161 through a shortlongitudinal slot 166 and runs from there to and through one end 167 ofthe rail 155. Preferably, the housing 64 is formed by molding plastic toyield two rigid complementarily-shaped pieces that are joined togetherover ends 163 and 167 of the rails 153 and 155 and the near short edge168 of the pouch 150. Together, the housing 64 and the rails 153, 155form a generally quadrilateral frame on which the pouch 150 issupported.

Many materials and processes may be used to construct the warmingcassette 60. For example, with reference to FIGS. 5 and 6, we assemblethe fluid container 62 from sheets of laminated material which include alayer of polyethylene material on a layer of polyester; we use railsmade of molded polyethylene; and we assemble the housing 64 using piecesmade of a molded acrylic, polycarbonate, or blended medical gradeplastic such as Cyrolite®. The sheets are oriented with the polyethylenelayers facing and the rails are disposed between the polyethylene layersin the orientations seen in FIGS. 5 and 6. The seals 151 and 152 in thiscase may be formed by heat applied through the polyester layers. Becauseof difficulty in sealing the polyethylene rails to the polycarbonatehousing, we use compliant sleeves 169 and 170 made of polyvinyl chloride(PVC) to attach the ends 163 and 167 of the rails 153 and 155 to thehousing. In this regard, the sleeves 169 and 170 are contained withinthe housing 64 and their outside surfaces are sealed with solvent tocomplementary structures in the housing. Barbs formed on the ends 163and 167 of the rails 153 and 155 mechanically seat against the interiorsurfaces of the sleeves 169 and 170, attaching the rails 153 and 155 tothe housing 64 in the positions shown in FIG. 6.

Infusate flow through the warming cassette 60 is shown in FIG. 6. Theend 163 acts as the input port 69 of the warming cassette 60. Infusateenters the warming cassette through a tube (not shown) in fluidcommunication with the end 163, flows through the central passageway 162in the rail 153 and exits the rail 153 through the slot 165. Infusateflows through the inlet 160 wherefrom it fans out into a broad thinsheet that extends across the laminar flow path 67 that flows toward thehousing 64. As the sheet of infusate approaches the housing 64, it isfunneled toward the outlet 161 by the curve 172 formed by the contour ofthe seal 151. The infusate flows out of the laminar flow path 67 throughthe outlet 161 into the short passageway of the rail 155 via the slot166. The infusate flows out of the short passageway of the rail 155through the end 167 and into the bubble trap 80 in the housing 64 of thewarming cassette 60. The infusate flows through the bubble trap 80 andthe valve 82, to and out of the output port 71.

Heating Unit

With reference now to FIGS. 7, 8, 20, and 21, the warming cassette 60 isshown inserted into a heating unit 180 supported in the infusion unit10. The heating unit 180 includes two opposed heating plates 182 and 184that define a narrow laminar space within which the fluid container 62is seated. Preferably, the heating plates 182 and 184 are formed of lowthermal resistance aluminum anodized with a hard coat. The heatingplates 182 and 184 conduct heat generated by a pair of resistanceheaters on the outside surfaces of the heating plates. One such heater183 is best seen in FIG. 20. The resistance heaters 183 may comprise,for example, laminated silicone resistance heaters, or equivalentsthereof. The heating plates 182 and 184, with the heaters 183 mounted tothe outside surfaces thereof, are conventionally mounted in the pedestal22 of the infusion unit 10. As seen in FIGS. 7-9, the heating plate 182has elongate parallel grooves 186 near its lateral edges which faceopposing elongate parallel grooves 188 in the heating plate 184. As bestseen in FIGS. 8 and 9, the facing grooves 186, 188 form elongateparallel channels that accommodate the rails 153 and 155 and guide thewarming cassette 60 to and from correct seating as it slides between theheating plates 182 and 184. As seen in FIGS. 8 and 9, the warmingcassette 60 has a thin, but relatively elongate aspect (when vieweddistal end on) so as to be slidably inserted through the slot 34 in thebezel 32 with the fluid container 62 sandwiched between and inheat-conducting contact with the heating plates 182 and 184, andslidably extracted therefrom. As seen in FIG. 9, the laminar flow path67 is sandwiched between and in close abutting contact with the heatingplates 182 and 184 when the warming cassette 60 is installed in theinfusing unit 10, thereby eliminating the need for an intermediarymedium to transport heat to the warming cassette 60.

In FIG. 7, a pair of opposing shallow transverse channels 190 formed inthe surfaces of the heating plates that face the fluid container 62 runfrom an edge of the heating plate surface. Corresponding ends of thechannels are near the location of the slot 165 in the rail 153 when thecassette 60 is seated in the electrical heating unit 180. The pressureof infusate flowing out of the slot 165 forces opposing strips of thefluid container 62 into conformance with the channels 190, therebyforming an input manifold through which infusate can spread into thelaminar flow path 67. Similarly, a pair of opposing shallow transversechannels 192 formed in the surfaces of the heating plates that face thefluid container 62 cause the formation of an output manifold in thefluid container 62 that channels infusate out of the laminar flow path67 into the slot 166 in the rail 155.

FIG. 7 also shows monitoring and extraction elements of the heatingplates 182 and 184. A pair of opposing through holes 193 and 194 areformed in the heating plates 182 and 184 for positioning heat sensors(not seen) in the pair of opposing shallow transverse channels 192 thatcause the formation of an input manifold in the fluid container 62. Atthese opposing locations, the temperature of infusate flowing out of thewarming cassette may be measured. Two pairs of opposing through holes195 are formed in the heating plates 182 and 184 for channeling jets ofpressurized air against the sides of the fluid cassette in order todislodge the warming cassette from the heating plates 182 and 184. Inthis regard, when the flow of infusate ceases, a sheet of infusate fillsthe fluid container 62, pressing the sides of the fluid containeragainst the opposing surfaces of the heating plates 182 and 184. Surfacetension and friction between the fluid container 62 and heating plates182 and 184 can make it difficult to dislodge warming cassette 60. Jetsof pressurized air through the holes 195 force infusate out of the fluidcontainer 62, thereby breaking the surface tension and reducing thefriction, making it easier to extract the warming cassette 60.

With reference to FIGS. 6 and 9, important benefits of the warmingcassette construction will be appreciated. The dry heat mode of warminginfusate shown in these figures eliminates the need for a fluid such aswater to transport heat to the infusate. At the same time, the broad,unidirectional laminar flow through the heat exchanger that isconstrained between the heating plates minimizes flow path resistance byeliminating successive curves and reverses in the direction of flow.Presuming a maximum width of the laminar flow path that is dictated bydesign constraints, it is, of course, possible to reduce flow resistancefurther by increasing the spacing between the heating plates, but thisalso reduces the rate of heat transfer from the heating plates to theinfusate. Thus, there are optimal balances between fluid flow and heattransfer that can be achieved for various applications of the warmingcassette construction illustrated and described above.

Bubble Trap and Shut Off Valve

For the purposes of the following explanation, the housing 64 has afront face, seen in FIGS. 10 and 12A, that is visible to an operatorwhen the warming cassette 60 is installed in the infusion unit 10, and arear face, seen in FIG. 12B that faces the mounting block 36 when thewarming cassette 60 is installed in the infusion unit 10. With referenceto FIG. 10, after infusate has been warmed in the heat exchanger, thebubble trap 80 separates and collects air and bubbles from the infusateas it streams in a flow path (a “trap flow path”) through the bubbletrap 80, and vents air through a vent. If a threshold level of air isdetected in the bubble trap 80, the valve 82 closes, thereby stoppingthe flow of warmed infusate to the patient line. Preferably, the bubbletrap 80 and valve 82 are integral parts of the housing 64. That is tosay, the molding process with which the components of the housing 64 ismade forms the structural components of the bubble trap 80 and valve 82in the housing components, so that the components of the bubble trap 80are assembled and contained within the housing 64 when the molded halvesof the housing are joined. This construction is preferred, but shouldnot be limiting. For example, a bubble trap can be constructedseparately and placed within the housing 64 as the housing is assembled.

The bubble trap 80 includes a trap flow path designed for high flowrates, that is, flow rates of 30 L/hr, and higher. Preferably, thebubble trap operates with fluid flow rates in the range from 0 to 70L/hr, or higher, through the trap flow path. The trap flow path isconstructed to separate bubbles from the infusate in a succession ofstages. The stages include, in sequence, a flow velocity reductionchamber (“reduction chamber”), a separation chamber, a laminar flowchamber, and an outlet chamber.

With reference to FIG. 10, the reduction chamber 200 is in fluidcommunication with the central passageway of the rail 155, so thatinfusate flows out of the end 167 of the rail 155 into the reductionchamber 200. The reduction chamber 200 has a hook-shaped cross sectionthat increases in width from the end 202 to the top portion of the hook.When the warming cassette is oriented vertically in the infusion unit 10as shown in FIG. 2, the end 202 is in the bottom of the bubble trap 80.In this case, the top portion of the hook bends downwardly at 203 to theseparation chamber 205. In some aspects, a baffle 206 may be provided tochannel infusate flow into the separation chamber 205. With reference toFIGS. 10 and 11, the trap flow path narrows substantially in thetransition from the separation chamber 205 to the laminar flow chamber207. As best seen in FIG. 11, the laminar flow chamber 207 has a narrowcross section with an outer side 64 o in the front face of the casing 64and an opposing inner side 64 i in the rear face of the casing 64.Referring to FIGS. 10, 13, and 16, a disc-shaped hydrophobic membrane209 is welded to the inside surface of the outer side 64 o, spaced apartfrom apertures 210 through the outer side 64 o. When infusate flowsthrough the bubble trap, the hydrophobic membrane 209 is continually incontact with the infusate as it flows through the laminar flow chamber207. Advantageously, the accelerated flow of infusate through thelaminar flow chamber keeps bubbles from sticking to the surface of, andclogging, the hydrophobic membrane 209. As best seen in FIGS. 10 and12B, first and second sensor couplers 37A and 38A are supported on theoutside surface of the inner side 64 i. Presume that the warmingcassette 60 is oriented vertically in the infusion unit 10 as shown inFIG. 2. In this case, as shown in FIG. 10, the hydrophobic membrane 209is positioned above and upstream of both sensor couplers 37A and 38A,and the first sensor coupler 37A is positioned above the second sensorcoupler 38A. As seen in FIGS. 10 and 11, the trap flow path transitionsat 214 to the outlet chamber 215. As best seen in FIG. 10, infusateflows out of the outlet chamber 215 through a thimble-shaped hydrophilicscreen 217 into a short riser 219 by which it enters one side of acircular valve chamber 220 that is in fluid communication with theoutput port 71. Preferably, the outlet chamber 215 is widened withrespect to the laminar flow path in order to reduce flow velocity of theinfusate through the hydrophilic screen 217 so that bubbles will not bepushed through the screen and can rise up off to the air pocket formingin the top portions of the bubble trap 80.

In some instances, the housing 64 may be transparent in order to enablean operator to see and judge bubble trap operation through theseparation chamber 205; in these instances, as best seen in FIG. 11, thehousing 64 may bulge outwardly at 208 thereby to enable the operator toclearly see the infusate level in the bubble trap 80. For example, theoperator may view the cascade of infusate flowing over the bend 203 tovisually ascertain infusate flow and judge the flow rate.

With reference to FIG. 10, infusate flows into the bubble trap 80 fromthe end 202 of the reduction chamber 200. Presume that the warmingcassette is oriented vertically in the infusion unit 10 as shown in FIG.2. In this case, as the bubble trap 80 is primed with infusate throughthe end 202, the infusate wells up from the bottom of the bubble trap,thus ensuring that it does not form a free jet as it enters the bubbletrap. As infusate flows through the reduction chamber 200, theincreasing width of the reduction chamber expands and slows the infusatestream. The slowed infusate stream rises in the hook shape of thereduction chamber 200 and flows over the bend 203, cascading from theupper portion of the reduction chamber 200 into the separation chamber205. If used, the baffle 206 is positioned to confine the cascadinginfusate stream downwardly, through a channel along the wall 211, intothe bottom of the separation chamber 205. The cascade of infusate intothe separation chamber 205 enters the widest portion of the bubble trap80, but encounters the sharp reduction in cross section in thetransition to the laminar flow chamber 207, which sets up arecirculating flow pattern in the separation chamber 205. The narrowcross section in the laminar flow chamber 207 accelerates the infusateand forces it once more into a sheet that traverses the laminar flowchamber 207 across the hydrophobic membrane and past the sensor couplers37A and 38A. The laminar stream of infusate enters the outlet chamber215, where it is funneled into the short riser 219, which narrows andfurther accelerates the infusate flow and turns it toward the valvechamber 220 from which the infusate stream flows out of the warmingcassette through the output port 71.

In FIG. 10, as the infusate flows through the trap flow path, thebuoyancies of air boluses and large bubbles in the infusate pull themfrom the infusate stream as the stream flow slows through the reductionchamber 200. These large-diameter bubbles are collected in the reductionchamber 200. Thus, for example, bubbles 222 having diameters in therange of 1 to 3 mm, and larger, will separate from the stream ofinfusate and rise to be collected in the hooked upper portion of thereduction chamber 200. As the large bubbles rise and collect, theyburst, which causes an air pocket 223 to form. As the infusate streamturns at the bend 203 and cascades into the separation chamber 205,bubbles remaining in the stream are circulated in the eddy of infusatein the separation chamber. This lengthens the dwell time of bubbles inthe separation chamber 205, thereby increasing the likelihood that theywill rise and burst, adding to the air pocket in the reduction chamber200. Some small (1 mm diameter, for example) bubbles may be entrainedinto the eddy in the separation chamber 205 from foam at the borderbetween an air pocket and the infusate; these bubbles tend to remaintrapped in the eddy without passing to the laminar flow chamber 207. Asthe infusate stream passes through the laminar flow chamber 207 to theoutlet chamber 215, very small bubbles remaining in the infusate areprevented by the hydrophilic screen 217 from leaving the outlet chamber215. These small bubbles stick to the surface of the screen 217, but arenot drawn through. Over time, multiple bubbles coalesce on thehydrophilic screen 217, forming larger bubbles with enough buoyancy tolift off the hydrophilic screen and rise to the top of the bubble trap80. Air expelled with infusate from a bag may also enter the trap. Asair accumulates in the top of the bubble trap 80, it is vented from thetrap through the air vent 81 by the hydrophobic membrane 209 and theapertures 210. If the level of collected air in the bubble trap reachesthe sensor couplers 37A and 38A the valve 82 is closed and infusate isstopped from flowing out of the warming cassette 60.

The hydrophobic membrane 209 provides preferential flow of gases overliquids and therefore draws air from the bubble trap 80 and releases itto the ambient atmosphere. Thus, the hydrophobic membrane 209 serves asa vent through which air is eliminated from the bubble trap 80. Arepresentative construction of the hydrophobic membrane is shown in FIG.13, wherein a 2-3 mil thick hydrophobic membrane constituted of apolymer material, preferably an expanded polytetrafluoroethylene (ePTFE)disc 225 having a nominal 0.45 micron pore size has a polyester nonwovenbacking 227. We have obtained such a hydrophobic membrane from W.L. Gore& Associates. The membrane 209 has a disc-like shape and may be glued,bonded, or welded directly to the inner surface of the outer side 64 o,with the polyester backing 227 in contact with the inner surface and thehydrophobic material facing the laminar chamber 207. The outer side 64 oof the laminar flow chamber is outwardly contoured to provide acylindrical ledge 229 on its inner surface to position and support themembrane 209, and a cylindrical vent chamber 230 to collect air passedthrough the membrane 209 from the bubble trap 80. Intermittent ridges232 in the chamber 230 support the membrane 209 against the pressure ofinfusate flowing through the bubble trap 80, but do not impede thecirculation of air in the vent chamber 230. Vent holes 210 (best seen inFIGS. 10 and 12A) permit air to pass from the bubble trap 80, throughthe outer side 64 o, to the ambient atmosphere. With reference to FIGS.12A and 13, an umbrella-shaped silicone check valve 234 is mounted onthe outer surface of the outer side 64 o by a central mounting hole 236.The outer rim 238 of the check valve 234 covers the openings 210. Whenthe pressure of the air collected in the vent chamber 230 exceedsatmospheric pressure, the outer rim 238 yields and collected air passesthrough the openings 210 to the ambient atmosphere.

With reference to FIGS. 12B and 15, the sensors 37 and 38 sense thelevel of fluid (air and infusate, for example) and enable the detectionof air in the bubble trap 80 for the purpose of controlling the flow ofinfusate. In some aspects, the sensors 37 and 38 may operateultrasonically. In these instances, accurate sensing requiressuppression of an echo reflected from an impedance mismatch such assolid/air transition at the rear face of the housing 64, which faces thesensors 37 and 38. The sensor couplers 37A and 38A mounted to thehousing eliminate reflections of transmitted ultrasonic pulses from therear face of the housing 64. A representative construction of the sensorcouplers 37A and 38A is shown in FIGS. 14A and 14B. The sensor couplers37A and 38A may be formed in a piece 240 of a relatively soft, butdurable material that has a high transmissivity at ultrasonicwavelengths. The piece 240 has a flat, planar front surface 242 and arear surface 244 on which domes 246 may be formed to increase couplingeffectiveness. The domes 246 constitute the sensor couplers 37A and 38A.The front surface 242 of the piece 240 is adhered, bonded, or welded tothe outside surface of the inner side 64 i, adjacent the laminar flowchamber 207. Presume the warming cassette 60 is seated in the infusionunit 10 as shown in FIG. 2; then, as seen in FIG. 15, the faces of thesensors 37 and 38 are in intimate pressing contact with the sensorcouplers 37A and 38A. The material of which the sensor couplers areconstructed minimizes or eliminates reflection of transmitted ultrasonicpulses from the outside surface of the rear face of the housing 64 andpasses echoes reflected from the inside surface of the front face of thehousing 64. It is advantageous to have the sensor couplers 37A and 38Amounted to the housing 64 because the material of which the aperturesare made can be less durable than if mounted to the mounting block 36 orthe sensors 37 and 38. This is because the piece 240 has to undergo onlya single use that occurs when the warming cassette is inserted in theinfusion unit 10. The domes 246 formed on the piece 240 allow thematerial of which it is formed to displace more easily when in responseto sensor contact, which makes the material appear even softer than ifthe sensors 37 and 38 displaced a flat planar surface. We use a sensorcoupler piece 240 formed of injection-molded thermo-plastic elastomer(TPE) 5.175 mm thick, 30 durometer, shore A.

A representative construction of the hydrophilic screen 217 that filterssmall bubbles from the infusate path in the outlet chamber 215 of thebubble trap 80 is shown in FIG. 16. The hydrophilic screen 217 isconstituted of a 263 micron nylon mesh 250 supported on a molded plasticsupport 252. We have obtained such a screen from GVS Filter Technology,Rome, Italy. The hydrophilic screen has an open end 254, and an oppositeend (not visible in FIG. 16) which may be closed by an element of themolded plastic support 252. Referring again to FIG. 10, the open end 254of the resulting thimble-like structure is glued, bonded, welded, orfitted to the outlet structure of the outlet chamber 215, in fluidcommunication with the inlet to the riser 219.

With reference to FIGS. 12B, and 17, the valve 82 includes the circularvalve chamber 220, a valve membrane 260, and a seating ring 262. Thevalve membrane 260 is disposed over a second side of the circular valvechamber 220 and held thereto by the seating ring 262. When the piston 41is retracted, the valve 82 is open; to close the valve 82, the actuator40 is activated, which throws the piston 41 against the valve membrane260, forcing the membrane against the open end 264 of the riser 219.This prevents infusate from flowing into the circular valve chamber 220and out of the output port 71. Preferably, the valve membrane 260 may beformed of silicone or any other durable, flexible material that iscompatible with blood. We have obtained such a silicone valve membranefrom Liquid Molding Systems, Midland, Mich. Alternately, the valve 82could be constituted of a rigid, electromechanically-actuated valve,such as a quarter-turn stopcock.

Heat Exchanger Installation and Retention

Use and operation of the high flow rate infusion unit are simplified byan interlocking mechanical interface between the infusion unit and heatexchanger that enables an operator to quickly and easily install theheat exchanger, bubble trap, and shut off valve in a single act. Bysliding the heat exchanger into position between the heating plates, theoperator positions the laminar flow path for heat exchange, locates thebubble trap for monitoring by the sensors 37 and 38, and orients theshut off valve for operation.

Considering the exemplary embodiment of the heat exchanger, when thewarming cassette is installed in the high flow rate infusion unit,various elements of the warming cassette 60 and the infusion unit 10cooperate to seat the warming cassette and to enable the infusion unitto control the flow of infusate. In this regard, with reference to FIGS.18 and 19, the housing 64 engages the mounting block 36 and rests on thebezel 32. The warming cassette is thus retained in place against themounting block 36, and supported by the mounting block 36 and the bezel32, when installed. In this position, the fluid container 62 is alignedin operable engagement with the heating plates, the sensor couplers 37Aand 38A are aligned in operable engagement with the sensors 37 and 38,and the valve membrane 260 is aligned in operable engagement with thepiston 41.

With reference to FIG. 19, a warming cassette 60 is partially installedin the infusion unit 10, with its distal end 61 having been received inthe slot 34 and its rails in the circular enlargements 270. As downwardpressure is exerted on the warming cassette 60, the housing 64 movestoward the bezel 32. Construction details of the bezel 32 are shown inFIG. 18. The bezel 32 is fixedly mounted on flat planar area of therecessed surface portion 30, oriented transversely to the pedestal 22.The mounting block 36 is fixedly mounted in the recessed surface portion30, disposed substantially perpendicularly to and abutting an insideedge of the bezel 32. The slot 34 in the bezel 32 is orientedtransversely to the pedestal 22 and in parallel with the major surface42 of the mounting block 36. The slot 34 is aligned with the narrowlaminar space between the heating plates and includes a diamond shaped,oval, or round enlargement at each end to accommodate the rails of awarming cassette. Each circular enlargement is aligned with the elongateparallel channels formed by the elongate parallel grooves of the heatingplates (See FIGS. 8 and 9). One such circular enlargement 270 is seen inFIG. 18. The bezel 32 is therefore constructed to receive a warmingcassette, distal end first, in the slot 34, with the rails of thewarming cassette received in the circular enlargements 270 so as toguide the fluid container of the warming cassette into the narrowlaminar space between the heating plates for seating therebetween. As isevident from FIG. 18, the bezel 32 forms a raised frame to support thehousing 64, and includes a forward edge 271 that slopes downwardly andaway from the slot 34.

As seen in FIGS. 12B and 18, a thin flange 272 projects from an edge 273of the mounting block 36; the front surface of the flange 272 forms aportion of the major surface 42. A sensor 274 is mounted adjacent therear side of the flange 272, on the edge 273. Preferably, the sensor 274is an inductive proximity sensor. Two tabs 275 protrude outwardly inopposite directions from the bottoms of the lateral edges of themounting block 36. One tab 275 is seen in FIG. 12B; its opposite is seenin FIG. 19. The rear side of the flange 272 has a recess with aprojecting notch 276 near the edge 273. FIG. 15 shows that the flange272 is wedge-shaped in its upper extent 277. A pair of retaining pins isfixedly mounted in opposing relationship to the opposing sides of therecessed surface portion 30 of the infusion unit pedestal 22. One of theretaining pins 278 can be seen on one of the opposing sides 279 in FIG.18. As seen in FIGS. 12B, 18 and 19, a sloped elongate trench 280 withrounded ends in the major surface 42 surrounds the locations of thesensors 37 and 38, which protrude beyond the plane of the major surface42, toward the housing 64. When a warming cassette is installed, therear face of the housing 64 is slightly separated from the major surface42. When the housing engages and latches to the mounting block 36, thesensor couplers 37A and 38A on the rear face of the cassette housingalign with and contact the faces of the sensors 37 and 38, and retainthe sensors in engagement while the warming cassette is installed in theinfusion unit. FIGS. 12B and 18 also show the actuator 40 mounted to theback of the mounting block 36 aligned with a through the hole 282through which the piston 41 is moved back and forth.

A cutout 286 with an upper edge 288 in the rear face of the housing 64is visible in FIG. 12B. The cutout 286 is shaped to accommodate theshape and dimensions of the mounting block major surface 42. As bestseen in FIGS. 12B and 15, inner side 64 i of the housing 60 is insetfrom the cutout 286. FIG. 12B shows a slot 290 in the upper edge 288 anda metallic strip 291 mounted in the housing adjacent the slot 290. FIG.12B also shows an upper flexible tab 292 formed in the upper edge 288.Two spaced-apart flexible tabs inset from the upper edge 288 are formedin the lower portion of the housing 64. One of the tabs 293 is seen inFIG. 12B.

With reference to FIG. 19, the warming cassette 60 is installed in theinfusion unit 10 by orienting the rear face of the housing 64 to facethe infusion unit 10 and then sliding the distal end 61 into the bezelslot 34, with the rails 153 and 155 received in the circularenlargements 270. With reference to FIG. 15, as the warming cassette 60slides home, the upper edge 288 of the housing cutout 286 engages andslides along the wedge-shaped upper extent 277 on the back of themounting block's flange 272, and (as shown in FIG. 20) the front face ofthe housing slides along the inner sides of the retaining pins 278. Theinner side 64 i is inset from the cutout 286 and spaced by a small gapfrom the major surface 42 of the mounting block 36. As the upper edge288 of the housing cutout approaches the ledge 273, the slot 290 in theupper edge 288 aligns with and accommodates the sensor 274 on themounting block 36, and metallic strip 291 is located near the sensor274. With reference to FIG. 12B, the flexible tab 292 in the upper edge288 aligns with and latches to the projecting notch 276 on the back ofthe flange 272, and further movement of the warming cassette 60 isstopped when the cutout upper edge 288 meets the ledge 273 of themounting block 36, and the lower edge of the housing 64 meets the uppersurface of the bezel 32. The warming cassette is now installed in theinfusion unit 10 (as shown in FIG. 20), with the fluid container 62seated between and in contact with the heating plates (see FIGS. 7-9),and with the sensor couplers 37A and 38A aligned and in contact with thesensors 37 and 38, and the valve membrane 260 aligned with the piston 41(see FIGS. 12B, 20, and 21). The warming cassette is guided by the pins278 into retention in the installed position by engagement between theupper edge 288 and the rear of the flange 272, engagement between thetabs 293 and the tabs 275, and locking of the flexible latch 292 to thenotch 276. The warming cassette is released by disengaging the flexiblelatch from the notch while pulling upwardly on the housing 64.

Audible and tactile feedback indicating that the warming cassette iscompletely seated is provided to an operator by the latching action ofthe tab 292 and the stopping of the housing 64 by the ledge 273. As bestseen in FIGS. 20 and 21, the lower front and side edges of the housing64 surround and shroud the bezel slot 34 so that the housing 64 shroudsthe slot 34, enclosing and covering it to prevent fluid that might reachthe bezel 32 from leaks in infusate bags, IV lines, or the housing 64from flowing thereinto.

Infusion Unit Subsystems

The high flow rate infusion unit includes an electronic controlsubsystem with input, logic, and output elements that receive commandand sensor inputs, process the inputs to set or change the controlconfiguration of the unit during operation, and produce outputs thatimplement the current control configuration. The electronic controlsubsystem is assembled from conventional electrical, electronic, andelectromechanical components mounted conventionally by means of printedcircuit boards and structural elements in the neck and pedestal of theinfusion unit. The electronic control subsystem is illustrated in FIG.22.

In FIG. 22, the electronic control system (“control subsystem”) 300includes a controller 302 having at least five logic blocks labeled#1-#5. Preferably, the controller 302 is assembled using discretecomponents conventionally mounted to one or more circuit boards.However, the controller 302 may also be assembled from programmableand/or programmed elements including general or special purposeprocessors, programmable logic arrays, and other equivalent components.Inputs to the controller 302 are received from a power supply 304 and abattery pack 306. The power supply operates conventionally, convertingAC mains power to various DC power outputs. The battery pack providesstandby DC power to operate the controller 302 and control subsystemcomponents in the event that operation of the power supply 304 isinterrupted. AC mains power is provided to operate the heaters 183through a power relay 308 and a solid state relay (SSR) 310. Both relaysmust be closed in order for AC power to reach the heaters 183. Openingeither relay will interrupt the supply of AC power to the heaters 183,thereby causing the interruption of heat supplied to infusate flowingthrough a warming cassette seated between the heating plates 182 and184. An operator interface 311 (including the control panels 77, 79 inFIGS. 1 and 2) provides means by which an operator can input commandsand means to output information to the operator.

With further reference to FIG. 22, logic block #1 of the controller 302executes a fail safe control function based upon comparison of atemperature measured by a thermistor 312 with a threshold temperature toturn off power to the SSR 310. The thermistor 312 measures a temperatureof the heating plate 182. If the measured temperature should exceed thethreshold temperature, the logic block #1 generates signals to open therelay 308, thereby blocking the provision of AC power to the SSR 310 andthus to the heaters 183.

With further reference to FIG. 22, logic block #2 of the controller 302mediates a temperature control function that is based upon a set pointtemperature and an input from a resistance temperature detector (RTD)314 that measures a temperature of the heating plate 184. In thisregard, a temperature-influenced resistance measured by the RTD 314 isprovided to a controller 316 and converted to a temperature value by thecontroller. The controller 316 executes a temperature control functionto maintain the measured temperature at a set point value by turning theSSR 310 on and off as needed to keep the measured temperature at the setpoint temperature. Control signals produced by the controller 316 arepassed to the SSR 310.

With reference to FIG. 7, the thermistor 312 is mounted in the hole 194in the heating plate 182, and the RTD 314 is mounted in the hole 193 inthe heating plate 184, opposite the thermistor. As is evident from thefigure, the thermistor 312 and RTD 314 are located in the transversechannels 192 formed in the surfaces of the heating plates that cause theformation of a manifold in the fluid container 62 that channels infusatefrom the laminar flow path 67 into the slot 166 in the rail 155. Thus,with accounting for heat transfer through the fluid container, thethermistor 312 and the RTD 314 effectively measure the temperature ofthe heated infusate as it enters the bubble trap. Thus, the controller316 operates to maintain the temperature of warmed infusate flowing intothe bubble trap 80 at the set point. In logic block #1, if thetemperature of warmed infusate flowing into the bubble trap 80 asmeasured by the thermistor 312 exceeds the threshold temperature (whichpreferably is the sum of the set point temperature and a predeterminedsafety margin), the power relay is signaled to shut off AC power to theSSR 310. For example, we have used a set point temperature of 42° C.,and a threshold temperature of 46° C.

In FIG. 22, logic block #3 responds to activation of a release button320 by an operator signaling that a warming cassette is to be extractedfrom the infusion unit. As seen in FIG. 18, the release button 320 islocated on the top of the pedestal 22, adjacent the recessed surfaceportion 30. Preferably, the release button is a manually operated, pushbutton switch, although it may also be embodied as a pressure activatedelectronic switch or a touch screen icon. Through logic block #3,activation of the release button 320 assists in releasing a warmingcassette from engagement with the infusion unit by dislodging thewarming cassette from the warming plates 182 and 184 and withdrawing thepiston 41 from contact with the valve 82. In this regard, the warmingcassette may be dislodged by activating an electronically controlledpneumatic valve 322 to release one or more jets of pressurized air whichpass through the holes 195 in the heating plates 182 and 184 seen inFIG. 7. The piston position is determined by the conditions ofelectronically controlled pneumatic valves 324 and 326.

Logic block #4 of the controller 302 seen in FIG. 22 monitors the sensor274 seen in FIG. 12B. When the sensor 274 senses close proximity of themetallic strip 291 (as would occur when the housing was seated on themounting block 36), it produces a signal interpreted as confirming thepresence of a warming cassette properly aligned with and seated in theinfusion unit 10. Alternately, when the sensor 274 senses closeproximity of the metallic strip 291, the signal produced may beinterpreted as confirming the presence of the bubble trap 80 and properalignment of the valve 82 with the actuator 40 in the infusion unit.With reference to FIGS. 18 and 22, in some aspects, a photosensor 328may be provided on the mounting block 36 to provide an initialindication of the presence of the housing 64 near the mounting block,following which the sensor 274 will respond to close proximity of themetallic strip 291 to provide an indication that the housing has beenproperly seated on the mounting block in the manner previouslyexplained. In this case, concurrent outputs from the sensors 274 and 328is interpreted as confirming correct installation of a warming cassettewith its fluid container seated between the heating plates. Logic block#4 also provides control signals for activating an electronicallycontrolled pneumatic valve 332 that controls pressure in an airreservoir (not seen).

Logic block #5 of the controller 302 seen in FIG. 22 receives andprocesses signals output by the ultrasonic sensors 37 and 38 thatindicate the presence of a fluid (air or infusate) in the bubble trap,and signals output by a Hall effect sensor 330 in the actuator 40 thatindicates the position of the piston 41. As an additional safetymeasure, logic block #5 provides control signals for activating theON/OFF function of the pressure infusers.

The high flow rate infusion unit includes a pneumatic subsystem withelements that receive signals from the electronic control subsystem 300indicating the control configuration of the unit during operation, andrespond to the inputs by setting or changing the operational pneumaticconfiguration. The pneumatic subsystem also includes sensors thatprovide signals to the electronic control subsystem 300. The pneumaticsubsystem is assembled from conventional pneumatic components mountedconventionally by means of structural elements in the neck and pedestalof the infusion unit.

With reference to FIG. 23, the pneumatic subsystem 360 includes a maindistribution channel 370. Pressurized air is provided to thedistribution channel 370 from dual pumps 373, operating in parallel, viacheck valves 375. The dual pump configuration is preferred for enhancedperformance under normal operating conditions and also for safetyreasons. Both pumps operate while the infusion unit is warming infusate;if either pump fails during infusion, the remaining pump has thecapacity to carry on the operations necessary to keep the pneumaticsubsystem operating.

With further reference to FIG. 23, pressurized air in the distributionchannel 370 flows to the electronically controlled three way valves inthe pressure infusers 18 through a pressure regulator 376; pressurizedair in the distribution channel 370 flows through a check valve 378 tothe electronically controlled valves 324 and 326 which are preferablythree way valves; and pressurized air in the distribution channel 370flows to the electronically controlled valve 322 which is preferably athree way valve. The valve configuration in the pressure infusers 18 isnot limiting; many other configurations may be used.

With reference to FIGS. 4A, 4B, and 23, logic block #5 (FIG. 22) allowsthe pressure infusers 18 to turn ON/OFF, or not. In this regard, eachthree way valve 100 in the pressure infusers 18 is controlledelectronically via logic block #5 in the controller 302 to connecteither the ambient atmosphere or the distribution channel 370 to itsassociated dump valve 99. When a pressure infuser 18 is operated, itsthree way valve 100 is operated to connect the distribution channel 370to the associated dump valve 99, which causes pressurized air to inflatethe associated bladder 103, thereby forcing infusate from a bag B in thepressure infuser. When the bag B is empty, when infusion is completed,or in other appropriate circumstances, the valve 100 is operated toconnect the ambient atmosphere to the associated dump valve 99, whichcauses pressurized air in the associated inflated bladder 103 to flowout of the dump valve to the atmosphere, thereby deflating the bladder103.

With reference to FIGS. 22, and 23, pressurized air flows through thecheck valve 378 into the reservoir 382. The three way valve 332 iscontrolled electronically via logic block #4 to connect the output ofthe reservoir 382 to either the ambient atmosphere or the inputs of thevalves 324 and 326. Preferably, the actuator 40 is a double actingpneumatic piston actuator conventionally operated by pressurized airprovided by the valves 324 and 326. The valves 324 and 326 are operated1800 out of phase by logic block #5 to position the piston 41 at anextended position against the valve membrane 260, which closes the valve82, or a retracted position away from the valve membrane 260, whichopens the valve 82. When the pumps 373 are turned off and the releasebutton 320 is operated, the states of the three way valves 324 and 326are configured by logic block #3 for withdrawal of the piston 41 to theretracted position and pressurized air from the pumps 373 and in thereservoir 382 is provided to the three way valves to move the piston tothe retracted position. If the piston 41 is in the retracted positionand the warming cassette 60 is extracted when the pumps 373 are turnedoff and the release button 320 is operated, the state of the three wayvalve 332 is set to vent the contents of the reservoir 382 to theambient atmosphere.

With reference to FIGS. 7, 22, and 23, the three way valve 322 iscontrolled electronically via logic block #3 in the controller 302 toconnect either the ambient atmosphere or the distribution channel 370 tothe holes 195 in the heating plates 182 and 184. When the release button320 is operated, the three way valve 322 is configured to connect thedistribution channel 370 to the holes 195, thereby jetting pressurizedair therethrough which breaks away surface tension and pushes fluid outof the fluid container 62. Otherwise, the valve 322 is configured toconnect the ambient atmosphere to, or to close, the holes 195.

Method of Operation

The high flow rate infusion unit 10 with the heat exchanger 12illustrated in FIGS. 1 and 2 may be operated according to a method shownin the flow diagram of FIG. 24. With reference to FIGS. 1, 2, and 24,the method of operation 380 preferably initiates from a power on state382 in which power is initially supplied to the infusion unit 10, withor without a heat exchanger 12 (for example, the warming cassette 60)installed. During initiation of operation, electronics, pneumatics, andlogic are tested. If anomalies are found, the method exits to a failuremode 384, where one or more status indicators are provided on theinfusion unit's operator interface. With power successfully turned on,the method checks for installation of the heat exchanger at 386. If noheat exchanger is installed, the method of operation ensures that thepiston 41 is retracted at 387 and operation suspends at 385 until a heatexchanger, installed with its planar flow path seated in the heatingunit, is detected. With a heat exchanger installed, correct operation ofthe piston 41 is validated at 388 by, for example, three successiveoscillations between the retracted and extended positions. Failure at388 causes the method to exit to a failure mode 384. Otherwise, themethod 380 branches to concurrently executing air management and heatercontrol loops.

In this explanation, air management is based on a test for the presenceof a fluid such as infusate or air in the bubble trap. In FIGS. 2 and24, during the air management loop 390, the bubble trap 80 is checked(by the sensors 37 and 38, for example) for the presence of air at 391In this regard, it is preferred that, when a heat exchanger 12 isinstalled, it will be connected to an infusate bag for priming.Preferably, but not necessarily, the bag will be located in a pressureinfuser 18. The heat exchanger 12 will be primed by gravitational flowof infusate to and through the laminar flow path. Air will be expelledthrough the bubble trap vent, permitting the priming infusate to flowinto and fill the bubble trap 80. While the heat exchanger 12 is beingprimed, air will be detected and the air management loop 390 willtransition through 392 and 393, keeping the valve 82 closed anddisabling the pneumatic subsystem from inflating the infusion bladders103 shown in FIGS. 4A and 4B (by configuration of the three way valves100, for example). When the bubble trap 80 has been filled to a level atwhich the sensors no longer detect air, the air management loop 390 willtransition through 394 and 395, opening the valve 82 and enablingoperator action for starting the pneumatic subsystem to inflate theinfusion bladders 103. That is to say, the controller 302 will open thevalve 82, but will not initiate inflation. Instead an operator isprompted by an alarm or other indication to use the operator interface311 to activate a pressure infuser. In this regard, the operator willinput a command via the interface 311 causing a pressure infuser toactivate. Thereafter, during infusion, the air management loop 390operates in response to the presence or absence of air in the bubbletrap by taking the appropriate transition from 391. In the transition391, 394, 395, 391, no action is required at 394 if the valve 82 is openor at 395 if the pressure infusers are enabled. When a bag of infusatehas been emptied or is near empty in one pressure infuser, the operatorreconfigures the Y tube set 73 to stream infusate from a full bag in theother pressure infuser. Using the interface 311, the operator will stopoperation of the pressure infuser with the empty bag and start operationof the other pressure infuser. In response to the stop/start indicationsfrom the operator, the controller 302 (FIG. 22) operates the three wayvalves 100 to deflate the bladder 103 in the stopped pressure infuserand to inflate the bladder in the pressure infuser with the full bag. Tocontinue infusion, the operator replaces the empty bag in the stoppedpressure infuser with a full one.

An important safety feature of the air control loop is realized inclosing the valve 82 and stopping infusion when air is detected. If thevalve 82 should leak under the pressure of the infusate when closed, airmight pass with leaking infusate through the closed but leaking valve.Deflating the active bladder relieves the pressure on the closed valve,thereby reducing, if not eliminating the risk of air leaking through theclosed valve.

With reference to FIGS. 22 and 24, the heater control loop 400 isinitiated at 402 by initiating the controller 316, turning on theheaters 183, and bringing the heating unit to the set point temperature.If turn on fails to execute properly, the method exits to a failure mode384. After successful turn on, control of heating plate temperature forset point operation is implemented by operation of the set pointcontroller 316. While the heating plates operate, the controller 302continuously checks the fail safe control function at 404. If thethreshold temperature is exceeded, the heating plates are turned off at406 and a failure mode 384 is entered.

With reference to FIG. 24, the method of operation continuously checksthe status of all infusion unit processes during all operations. Failuremodes are dealt with as appropriate to the particular circumstances offailure. In most instances, the controller 302 responds to a failuremode by deflating the balloons 103 in the pressure infusers 18, closingthe valve 82, and providing audible and visual indicators via theinterface 311. Operator action, such as selection of an “OFF” button orcondition to turn the infusion unit off when an infusion is terminatedand system operation is to be ceased will trigger power off status. Insome instances an operator may also select an “OFF” button or conditionwhen a heat exchanger is not installed in the infusion unit 10. Forthese cases, and in other appropriate circumstances, once power on hasbeen successfully completed, the method of operation 380 continuouslymonitors a power off test at 410. If a power off condition is active,the method terminates all currently active processes, including the airmanagement and heater control loops, and transitions to 412, testingwhether a heat exchanger is installed in the infusion unit 10. If a heatexchanger is not installed, the method ensures that the piston 41 isretracted at 413, and then completes action by transitioning to a poweroff state at 415 wherein all processes are terminated and power isturned off. If a heat exchanger is detected at 412, the method 380ensures that the valve 82 is closed and the pressure infusers aredisabled (if not already turned off by the operator) at 416 so thatinfusate flow to the patient line and to the heat exchanger is stopped.When the release button is activated at 418, the method retracts thepiston 41 at 419 and dislodges the heat exchanger at 420. In thisregard, for the warming cassette embodiment, dislodging at 420 includesoperating the pneumatic subsystem to jet compressed air through theholes 195 to disengage the fluid container 62 from the heating plates182 and 184. The method then transitions to the power off state at 415.

Air Sensing and Management

Preferably, air is sensed in the bubble trap by one or more sensorsmounted in the infusion unit 10; preferably, at least two such sensorsare used in order to provide redundancy, operational hysteresis, and arich logical control mechanism for air management. We have usedultrasonic sensors that operate like sonar devices by transmitting andreceiving pulses of ultrasonic energy. In particular, each of thesensors 37 and 38 may comprise a ceramic pulse echo sensor embeddedpotted, or screw mounted in a respective anodized hole through themounting block 36. In operation, each sensor sends out an ultrasonicpulse through a medium, and detects an echo of the pulse reflected backto the sensor off of an impedance mismatch, such as occurs at asolid/air interface. One source of such sensors is the Zevex AppliedTechnology Division, Salt Lake City, Utah.

As seen in FIGS. 12B and 25, the sensors protrude through the majorsurface 42 of the mounting block 36 and face the rear face 389 of thehousing 60, in contact with the sensor couplers 37A and 38A formed onthe material piece 240. Presume that the sensor 37 emits a pulse ofultrasonic energy. The sensor pulse enters the coupler 37A, and travelsthrough the material piece 240 and the rear face 430. Because of theinsignificant difference in impedance between the sensor coupler andhousing materials, no echo is produced by the outside surface of therear face 430. If the level of infusate is above the position of thesensor coupler 37A, the pulse travels through infusate in the housing tothe front face 431, and an echo is produced by the solid/airdiscontinuity at the outside surface of the front face. The front faceecho travels back, through the infusate, the rear face, and the materialpiece 240 and is detected by the sensor 37. If, however, the level ofinfusate is below the position of the sensor coupler 37A, thetransmitted pulse meets an impedance discontinuity at the solid/airinterface between the rear face of the housing and air in the bubbletrap, and an echo is produced by the rear face 431. The rear face echotravels through the material piece 240 and is detected by the sensor 37.Manifestly, the elapsed time to detect the front face echo is longerthan that for the rear face echo. The sensor 37 provides a signalindicative of the elapsed time on a conductor 432 to the controller 302.The signal is interpreted as indicating the absence or presence of air(or, conversely, the presence or absence of infusate) in the bubble trap80. The consequence of the difference in elapsed time is that absence ofa rear face echo is interpreted as the presence of infusate (or,conversely, as the absence of air), while detection of a rear face echois interpreted as the presence of air (or as the absence of infusate).Logic provided in the sensor utilizes a pulse window beginning with thetransmission of a pulse having a width wide enough for a pulse to travelto and from the front face. An echo received within the pulse window isinterpreted as indicating the presence of infusate (or the absence ofair); no echo received within the pulse window is interpreted asindicating the presence of air (or the absence of infusate). The sensor38 operates identically. This sensor arrangement provides a single pointof sensor contact for transmitting and receiving.

Preferably, air management in the bubble trap is based upon venting airthrough a hydrophobic membrane in contact with infusate flowing throughthe bubble trap. In FIG. 26, the transition 214 between the laminar flowand outlet chambers 207, 215 includes a downwardly angled wall 440. Thesensors 37 and 38 have fields of view through the sensor couplers 37Aand 38A into the laminar flow chamber 207. The level line 442 iscentered in the field of view of the sensor 37, and the level line 444is centered in the field of view of the sensor 38. The level line 442passes through the lower quadrant of the hydrophobic membrane 209, andthe level line 444 is parallel to the level line 442, below thehydrophobic membrane 209, but above the riser 219 through which infusateflows to the valve 82 and then to the output port 71. As air collects ina pocket in the upper reaches of the bubble trap, the border between theair pocket and infusate moves down the downwardly angled wall 440; whenthe border moves downwardly across the hydrophobic membrane 209, air isvented from the air pocket through the membrane. When the border betweenthe air pocket and infusate is above a level line 442 or 444, the sensor37 or 38 associated with the respective level line senses fluid; whenthe border is below a level line 442 or 444, the sensor 37 or 38associated with the respective level line senses air. An advantage ofthe sensor locations is that the increased velocity of the laminar sheetof infusate through the laminar flow chamber 207 sweeps bubbles from thefields of view of the sensors 37 and 38. This reduces the risk of eithersensor 37 or 38 producing false level indications in response tobubbles.

The preferred air management logic control mechanization for the sensorsdisposed with respect to the bubble trap as in FIG. 26 is shown in FIG.27; this logic represents an adaptation of the air control loop 360 ofFIG. 24 for the case of two sensors. The logic of FIG. 27 controls thestate of the valve 82 and enablement of the pressure infusers 18according to whether the sensors 37 and 38 report the presence ofinfusate or air in the bubble trap. Initially, the heat exchanger isprimed at 450, when the fluid container 62 and the bubble trap 80 areempty. The sensors 37 and 38 both report the presence of air at 460 and461, satisfying the test at 462. The valve 82 is closed and the pressureinfusers 18 are disabled at 463. The logic loops through 460, 461, 462and 463 until either sensor 38 or 37 reports the presence of infusate(or, conversely, no air). When the presence of infusate is reported at460 or 461 the valve 82 is opened and the operator is given anindication to activate inflation of a balloon in a pressure infuser 18at 464. Then both sensors are monitored for air. When both sensorsreport air, the valve 82 is closed and the operating pressure infuser isdeactivated at 463, and the logic again loops until infusate is reportedby either or both sensors as previously mentioned.

When the valve 82 is closed in response to the test at 462, the bubbletrap is again primed with infusate, which will rise in the bubble trap,first passing the lower sensor 38. In some aspects, the logic of FIG. 27may utilize a time delay to the negative exit of the test at 461,thereby prolonging the closure of the valve 82 while the bubble trapprimes. In these instances, the use of two sensors provides hysteresisin the operation of the valve 82.

Other air sensing and management configurations for the bubble trap 80are possible. One such configuration, shown in FIG. 28 as an adaptationof the bubble trap 80, uses a second solenoid driven valve 470 toisolate the air vent 81 in an air chamber 471 in order to keep thehydrophobic membrane 209 dry. If either of the sensors 37 and 38 sensesinfusate, the valve 82 is open. If both sensors 37 and 38 sense air, thevalve 82 is closed. If either of the sensors 37 and 472 senses thepresence of infusate, the valve 470 remains closed. If both sensors 37and 472 sense the presence of air, the valve 470 is opened. The pressureof infusate flowing into the bubble trap 80 from the fluid containerforces the air into the air chamber 471 where it is vented through thehydrophobic membrane 209. The level of infusate rises as air exits intothe air chamber 471, and the valves 82 and 470, respectively, open andclose when the sensors 37, 38, and 472 once again sense the presence ofinfusate. A third ultrasonic sensor to sense the contents of the bubbletrap 80 through coupler location 472 may be included in order to providegreater redundancy, a larger degree of hysteresis, and a richerfunctional set than the two sensors 37 and 38. One additional functionrealized by the addition of a third sensor is to open the valve 470 atsome intermediate infusate level while holding open the valve 82 inorder to vent air while continuing to deliver infusate to a patient.

Although a heat exchanger for high flow rate infusion unit has beendescribed with reference to a number of embodiments, it should beunderstood that various modifications can be made without departing fromthe principles of this specification, which are limited only by thefollowing claims.

1. A heat exchanger for heating fluids in a high flow rate infusionsystem, comprising: an inlet and an outlet; a fluid container with anear and distal ends, two edges, and a laminar flow path in fluidcommunication with the inlet; a housing attached to the fluid containerat the near end; a bubble trap in the housing in fluid communicationwith the laminar flow path; and a valve in the housing in fluidcommunication with the bubble trap and the outlet.
 2. The heat exchangerof claim 1, the fluid container including a pair of rails, each railpositioned in the fluid container near a respective edge thereof suchthat the laminar flow path is between the rails, and each rail includinga first end at the near end of the fluid container and held in thehousing.
 3. The heat exchanger of claim 2, wherein: a first railincluding a first passageway in fluid communication with the inletthrough the first end and a lateral opening near the distal endproviding fluid communication between the first passageway and thelaminar flow path; and, the second rail including a second passageway influid communication with the bubble trap through the first end and alateral opening near the near end providing fluid communication betweenthe second passageway and the laminar flow path.
 4. The heat exchangerof claim 3 wherein the first end of each rail is barbed, furtherincluding a pair of recesses in the housing, each first end held in arespective recess.
 5. The heat exchanger of claim 1, further including asensor coupler on an outside face of the housing, adjacent the bubbletrap.
 6. The heat exchanger of claim 5, the sensor coupler constitutedof a molded thermo-plastic elastomer piece with at least one dome formedthereon.
 7. The heat exchanger of claim 1, further including means on anoutside face of the housing, adjacent the bubble trap, for contacting aplurality of ultrasonic sensors.
 8. The heat exchanger of claim 7, themeans constituted of a molded thermo-plastic elastomer piece with aplurality of domes formed thereon.
 9. The heat exchanger of claim 1,further including an air vent opening through a first face of thehousing to the bubble trap.
 10. The heat exchanger of claim 9, whereinthe air vent is constituted of a vent chamber formed in the first face,a hydrophobic membrane on an inside surface of the first face, over thevent chamber, one or more holes through the vent chamber, and a checkvalve mounted to an outside surface of the first face, over the holes.11. The heat exchanger of claim 10, further including means on a secondface of the housing, opposite the first face, and adjacent the bubbletrap, for conducting ultrasonic measurement signals into and out of thebubble trap.
 12. The heat exchanger of claim 11, the means constitutedof a molded thermo-plastic elastomer piece with a pair of domes formedthereon.
 13. The heat exchanger of claim 1, wherein the valve isconstituted of a valve chamber formed in the housing, an opening in afirst side of the valve chamber in fluid communication with the bubbletrap, and a membrane closing a second side of the valve chamber.
 14. Theheat exchanger of claim 13, further including an air vent openingthrough a first face of the housing to the bubble trap.
 15. The heatexchanger of claim 14, wherein the air vent is constituted of a ventchamber formed in the first face, a hydrophobic membrane on an insidesurface of the first face, over the vent chamber, one or more holesthrough the vent chamber, and a check valve mounted to an outsidesurface of the first face, over the holes.
 16. The heat exchanger ofclaim 15, further including means on a second face of the housing,opposite the first face, and adjacent the bubble trap, for conductingultrasonic measurement signals into and out of the bubble trap.
 17. Theheat exchanger of claim 16, further including an air vent openingthrough a first face of the housing to the bubble trap.
 18. Acombination for heating fluids in a high flow rate infusion system,comprising: a heat exchanger constituted of: an inlet and an outlet; afluid container with a near and distal ends, two edges, and a laminarflow path in fluid communication with the inlet; a housing attached tothe fluid container at the near end; a bubble trap in the housing influid communication with the laminar flow path; and a valve in thehousing in fluid communication with the bubble trap and the outlet; and,a pair of opposed heating plates defining space therebetween forslidably receiving the fluid container, the heating plates including:means in the heating plates responsive to pressurized infusate in thelaminar flow path for causing input and output manifolds to form in thefluid container; and means in the heating plates for causing the fluidcontainer to be disengaged from the heating plates.
 19. The combinationof claim 18, further comprising means for heating the heating plates.20. The combination of claim 18, further comprising means for providingpressurized infusate to the inlet and a patient line connected to theoutlet.
 21. The combination of claim 20, further comprising means forheating the heating plates.